What is Job Control?

Shell Job Control Commands Quick Reference

Job State Machine

SIGTTIN: Background Process Reading from Terminal

SIGTTOU: Background Process Writing to Terminal (TOSTOP)

Code Example 1 — Program That Responds to SIGTTIN and SIGTTOU

/* sigttin_demo.c
 * Demonstrates SIGTTIN when a background process tries to read.
 * Run this in the background: ./sigttin_demo &
 * Then wait — it gets stopped by SIGTTIN.
 * Bring to foreground with: fg
 * Compile: gcc -o sigttin_demo sigttin_demo.c
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>

static void sig_handler(int sig)
{
    char msg[80];
    snprintf(msg, sizeof(msg),
             "\n[PID %ld] Caught signal %d (%s)\n",
             (long)getpid(), sig, strsignal(sig));
    write(STDERR_FILENO, msg, strlen(msg));
}

int main(void)
{
    /* Catch SIGTTIN and SIGTTOU so we can observe them */
    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = sig_handler;
    sigaction(SIGTTIN, &sa, NULL);
    sigaction(SIGTTOU, &sa, NULL);
    sigaction(SIGCONT, &sa, NULL);

    printf("PID=%ld  PGID=%ld\n",
           (long)getpid(), (long)getpgrp());
    printf("Checking if I am in foreground...\n");
    fflush(stdout);

    /* Open controlling terminal */
    int tty = open("/dev/tty", O_RDWR | O_NOCTTY);

    /* Try to read — this will cause SIGTTIN if we are in background */
    printf("Attempting read from /dev/tty...\n");
    fflush(stdout);

    char buf[64];
    ssize_t n = read(tty == -1 ? STDIN_FILENO : tty,
                     buf, sizeof(buf)-1);
    if (n > 0) {
        buf[n] = '\0';
        printf("Read: %s\n", buf);
    } else if (n == 0) {
        printf("EOF from terminal\n");
    } else {
        perror("read");
    }

    if (tty != -1) close(tty);
    return 0;
}
/*
 * $ ./sigttin_demo &
 * [1] 9100
 * PID=9100  PGID=9100
 * Attempting read from /dev/tty...
 * [1]+ Stopped   ./sigttin_demo      ← stopped by SIGTTIN
 *
 * $ fg
 * ./sigttin_demo
 * hello                               ← you type this
 * Read: hello
 */

Code Example 2 — Simulating fg/bg Job State Transitions

/* job_state_demo.c
 * Shows job states by printing what signals a process receives.
 * Run: ./job_state_demo &
 * Then try: fg, Ctrl+Z, bg, kill -STOP %1, bg
 * Compile: gcc -D_GNU_SOURCE -o job_state_demo job_state_demo.c
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>

static void handler(int sig)
{
    char msg[80];
    int n = snprintf(msg, sizeof(msg),
                     "[PID %ld] Signal: %s\n",
                     (long)getpid(), strsignal(sig));
    write(STDOUT_FILENO, msg, n);
}

int main(void)
{
    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = handler;

    /* Install handlers for all job-control signals */
    int sigs[] = { SIGTSTP, SIGSTOP, SIGCONT, SIGTTIN, SIGTTOU,
                   SIGHUP, SIGINT, SIGTERM, 0 };
    for (int i = 0; sigs[i]; i++) {
        /* Note: SIGSTOP and SIGKILL cannot be caught */
        if (sigs[i] != SIGSTOP)
            sigaction(sigs[i], &sa, NULL);
    }

    printf("Job state demo started.\n");
    printf("PID=%ld  PGID=%ld\n", (long)getpid(), (long)getpgrp());
    printf("Try: fg, Ctrl+Z, bg, kill -STOP %d, bg\n", (int)getpid());
    fflush(stdout);

    /* Loop printing heartbeat every 3 seconds */
    for (int tick = 1; ; tick++) {
        sleep(3);
        printf("  tick %d (still running, PID=%ld)\n",
               tick, (long)getpid());
        fflush(stdout);
    }
    return 0;
}
/*
 * Run in background, then interact:
 * $ ./job_state_demo &
 * $ fg                      → [PID 9200] Signal: Continued
 * Ctrl+Z                    → [PID 9200] Signal: Stopped
 * $ bg                      → [PID 9200] Signal: Continued
 * $ kill -STOP 9200         → job stops again
 * $ bg %1                   → [PID 9200] Signal: Continued
 */

Interview Questions — Section 34.7.1